Conventionally, a charge-imparting roller for use in an image-forming apparatus is formed of, for example, an epichlorohydrin rubber material to which an ion-conducting agent such as lithium perchlorate has been added.
Such a charge-imparting roller containing an ion-conducting agent has a drawback in that electric resistance of the roller varies considerably in accordance with variation in use conditions, potentially resulting in defective images.
Meanwhile, investigations have been carried out on a roller to which electrical conductivity has been imparted by use of carbon black, and a hybrid-type charge-imparting roller containing an ion-conducting agent and carbon black. Such a charge-imparting roller exhibits comparatively small environmental dependency, but has a drawback in that when carbon black particles are aggregated (dispersion failure), current leakage occurs along a pathway from an aggregation site to a photoreceptor, resulting in defective images with, for example, black lines.
Under the foregoing circumstances, a charge-imparting roller which can be used with consistent performance has been proposed. Specifically, electrical conductivity is imparted to the charge-imparting roller by use of carbon black, in which variation in electrical resistance is minimized to obtain a predetermined resistance value (see Patent Document 1).
However, the present inventors have found that image quality of actually obtained printed products cannot be predicted on the sole basis of variation in electrical resistance. In other words, even when variation conditions in electrical resistance are constant, quality of obtained images may vary.
Patent Document 1:
Japanese Patent Laid-Open (kokai) No. 2003-202750 (in claims and other sections)